1. Field of the Invention
The present invention relates, generally, to an improved extruder assembly for preparing a melt of molding material. More particularly, but not exclusively, the invention relates to improvements to a single screw compounding extruder wherein at least one wiping element is provided in a low pressure region of an extruder screw passageway, such as adjacent a devolatizing vent or an auxiliary additive feed port, to promote positive conveyance of the compounding material. The present invention has particular utility in the field of in-line compounding wherein apparatus and steps are required for the preparation of melts with entrained additives, for example long glass fiber reinforced polypropylene.
2. Background Information
A large percentage of plastics today are mixed with substantial levels of fillers (e.g. glass, carbon, and natural fibers, minerals, colorants, organic additives, etc.). These compounds are typically produced using a compounding extruder such as a twin screw intermeshing compounding extruder as described in U.S. Pat. No. 6,136,246 to Rauwendaal et al. Typically, the extruders are configured to produce pellets or billets of the molding compound that are subsequently used, in a substantially decoupled manner, as a feedstock into a typical injection or compression molding machine. Alternatively, the compounding extruder may be configured to feed the molten molding compound directly into the molding machine, this process is commonly known as ‘in-line compounding’ or ‘direct extrusion’. For example, U.S. Pat. No. 5,358,680, to Boissonnat et al., describes an inline compounding system that includes a twin screw compounding extruder, the extruder being configured for producing a molding compound of a thermoplastic that is blended with reinforcing fibers, and an injection molding machine that is configured to receive the molding compound directly from the extruder, the injection molding machine otherwise producing molded articles in the known manner.
Traditionally, the twin screw compounding extruder has been preferred over those with a single screw, despite some major shortcomings (e.g. considerably more expensive, screw wear, requirement for downstream pumping, etc.) because of their versatility and efficiency. Some inherent advantages of the twin screw extruder include a self-wiping of the outer surfaces of the intermeshed screw flights and shank, positive conveyance of molding materials in low melt pressure regions, good dispersive and distributive mixing, compact (i.e. short UD), flexible process control (i.e. owing to their starve-fed operation), and high throughput. Another reason why the twin screw compounding extruder has been favored has been the availability of modular screws and barrels whereby the extruder may be flexibly configured by simply adding or removing processing sections to achieve an optimal compounding process. Examples of commonly available barrel modules include primary feed, plain, vent, side stuff, and auxiliary feed sections. Similarly, some commonly available screw sections include various flight elements (i.e. feed, compression, venting, pumping, etc.), mixing elements, and zoning elements (i.e. isolate operations within the extruder).
By contrast, the single screw compounding extruder has traditionally been regarded as providing limited processing flexibility and a much narrower processing window. As discussed in detail in a recent paper entitled ‘New Single Screw Compounding Extruder’, by Chris Rauwendaal of Rauwendaal Extrusion Engineering Incorporated, this perception of the single screw extruder has been shaped by the commonplace practice of flood feeding the extruder, practical limitations on the extruder length, a lack of extruders configured with multiple feed ports, the use of screws and barrels of a unitary construction (i.e. configurations are not easily altered). This paper also proposes how many of these limitations can be addressed. For instance, a modular screw and barrel configuration, derived from the twin screw extruder art, could be used to provide additional processing flexibility. In addition, additional mixers and/or more efficient mixers, as described hereinafter, could be incorporated into the screw and/or barrel to minimize the extruder length required to achieve an acceptable level of additive distribution and dispersion. As well, the extruder may be configured to include a succession of vent/auxiliary ports along the barrel for a staged addition of the compound materials and volatile removal. A method for compounding in a single screw extruder is also proposed therein, with improved processing flexibility, that includes the step of starve feeding the extruder.
As introduced hereinbefore, there are a wide variety of mixer/kneader designs available in the art that are configurable in the single screw compounding extruder to improve its mixing efficiency and thereby minimize extruder length. The mixers may be formed or integrated directly onto the screw, barrel, or both. For instance, U.S. Pat. No. 5,932,159 to Rauwendaal, describes a mixer that integrates directly onto the screw. This mixer, commonly known as a ‘CRD mixer™’ (a trademark of Rauwendaal Extrusion Engineering Inc.), is realized by simply altering the screw flight configuration such that a front pushing face of the flight interacts with the inner surface of the screw passageway in the barrel to form a progressively narrowing passage through which material is forced into multiple regions of high elongational and shear stress whereby dispersive and distributive mixing is provided. As an alternative, a ‘Buss Ko-Kneader’ (a trademark of Coperion Holding GmbH) and generally described in the background section of U.S. Pat. No. 6,709,147, describes a mixer with elements that integrate into both the screw and barrel. In particular, this mixer requires the barrel of the extruder to include one or more axial rows of kneading elements that are arranged along an inner surface of the screw passageway in a high pressure stage of the extruder, as well as complementary shaped slots formed in the screw flights. In operation, the kneading elements move through the slots of the screw flights to create an efficient mixing action of the melt being pumped therebetween. The kneading elements, teeth or pins, are often press-fit, or threaded into the barrel of the extruder. It is also a common practice to integrate the kneading elements into a modular barrel portion that is configured to have a clam shell construction for sake of ease of installation and service.
Accordingly, there is strong potential for the single screw compounding extruder, when properly configured, to match the processing flexibility of the twin screw extruder but at much lower capital and operating costs. Despite this promise, there does remain at least one major problem. In particular, the lack of screw flight and shank wiping in a single screw compounding extruder can cause significant melt flow problems through the low pressure extruder stages (i.e. adjacent vents and auxiliary ports) when processing with at least one compounding material that exhibits a tendency to stick or otherwise coalesce along the boundary surface of the screw (i.e. screw root or flight). Unfortunately, this problem is particularly acute when the extruder is being starve fed.
Therefore, there is a need in the art to provide a wiper for wiping at least a portion of the screw flights and/or shank in the low pressure stages of a single screw compounding extruder adjacent the vent and/or auxiliary port to promote a proper conveyance of the molding material therethrough. This problem is also apparent in a single screw extruder (i.e. non-compounding) for the very same reasons.
When configured and used as intended, the mixing pins of the prior art ‘Buss Ko-Kneader’ (a trademark of Coperion Holding GmbH), as introduced hereinbefore, are located in extruder stages with sufficiently high melt pressure to ensure that the melt will continue to flow to the extent required to effect a thorough mixing/kneading of the melt as it is sheared between the pins and screw flight sections. However, nowhere is it known to in the art to use a similar configuration in the relatively low pressure stages of the extruder (i.e. near ambient pressure) for fulfilling the entirely different purpose of scraping and otherwise wiping the stalled compounding materials off of the screw flight and/or shank to promote its further interaction with screw and barrel portions to ensure an efficient transit of the low pressure stage extruder stage. In particular, nowhere is it known in the art to install screw wiping elements in the venting/entrainment extruder stages of a single screw compounding extruder.
A similar problem has been known to occur when feeding certain feedstock materials into the primary feed throat of the extruder wherein the feedstock becomes ‘tacky’ upon heating with a tendency to stick to the screw outer surfaces whereby the in-feed of molding material becomes impeded or blocked. A solution to this problem has been addressed in U.S. Pat. No. 3,929,323 to Smith. In particular, Smith describes a single screw extruder that includes an auxiliary feed screw that functions to wipe the screw flights of the extruder screw to ensure a positive conveyance of the molding material.
The following is a description of an experiment that was conducted on an in-line compounding molding system, the results from which are illustrative of the problem defined hereinbefore and that may now be advantageously addressed by implementing the improvements to the extruder in accordance with the embodiments of the present invention as described hereinafter.
With reference to FIG. 1 an in-line compounding molding system 10 is shown to include an experimental Husky™ (a registered trademark of Husky Injection Molding Systems Ltd.) P100/110 E100 two-stage extrusion/injection unit 14 that is coupled to a Husky™ (a registered trademark of Husky Injection Molding Systems Ltd.) GL300 PET clamp unit 12.
The clamp unit is typical for an injection molding system, and is shown to include a clamp base 18 with a stationary platen 16 securely retained to an end thereof, a clamp block 22 slidably connected at an opposite end of the clamp base 18, and a moving platen 20 arranged to translate therebetween on a set of tie bars 32 that otherwise interconnect the stationary platen 16 and the clamp block 22. As is known, the clamp unit 12 further includes a means for stroking (not shown) the moving platen 20 with respect to the stationary platen to open and close the injection mold halves 26, 27 arranged therebetween. A clamping means (not shown) is integrated within the clamp block 22 for generating a clamping force that is linked through a clamp column 24 to the moving platen for providing, in use, a clamping force between the mold halves 26, 27 during the injection of the melt of molding material, as is commonly known. The hot half of the injection mold 27 is mounted to a face of the stationary platen 16, whereas the complementary cold half of the mold 26 is mounted to an opposing face of the moving platen 20. The injection mold 25 is also shown to include a molding cavity 83 that is formed between the mold halves 26, 27, and a melt passageway 48G that passes through a sprue bushing portion 85 of the mold hot half 27 for interconnecting the molding cavity 83 to the melt passageway 48F of a machine nozzle 46.
The extrusion/injection unit 14 is a modified two-stage injection unit, and is shown to include a single screw compounding extruder assembly 38 that is arranged above, and in fluid communication with, an injection assembly 29. The extruder and injection assemblies 38 and 29 are both supported on a carriage 30 that is itself supported on ways 33 that are provided on the top bed of an injection unit base 28. Accordingly, the extruder and injection assemblies 38 and 29 can be moved together, relative to the clamp unit 12, for controllably coupling a machine nozzle 46 with the sprue of the mold hot half 27. A carriage cylinder 31 connecting the carriage 30 to the stationary platen 16 of the clamp unit 12 provides for the positioning of the carriage 30.
The extruder assembly 38 includes a screw drive assembly 36 that is configured for the rotation of an extruder screw 60 within a screw passageway 48A that extends along a cylindrical inner surface 49 of an extruder barrel 40. In operation, the rotation of the extruder screw 60 draws a first compounding material from a material hopper 34 into the screw passageway 48A, via a feed throat 58, as shown in FIG. 2A, and thereafter conveying the first compounding material through consecutive extruder stages, as shown in FIG. 2B and described in detail hereinafter, to a discharge end of the barrel 40. In the experiment the first molding material was a polypropylene thermoplastic. In contrast to a conventional two-stage injection unit where the screw is reciprocated, in use, by the drive assembly 36, to effect a transfer of an accumulated volume of the molding material in the extruder assembly 38 to the injection assembly 29 for subsequent injection, the present apparatus was operated in a mode whereby the extruder screw 60 is longitudinally fixed from reciprocating in the screw passageway 48A and wherein the steps of extrusion and injection are performed sequentially. The advantage of operating the extrusion/injection unit 14 in this manner will be evident in view of the description of the preferred embodiment of the present invention described hereinafter. The extruder assembly 38 also includes an auxiliary hopper 35 which functions to provide intermediate storage for a second compounding material which is introduced into the screw passageway 48A via an auxiliary port 59 that is arranged through the barrel 40 intermediate the primary feed throat 58 and the discharge end of the barrel 40. In the experiment the second molding material was reinforcing glass fibers 82. As is commonly known, the auxiliary port 59 also functions as a vent for the venting of volatile elements of the first compounding material. The extruder assembly 38 is also shown to include a barrel head 41 comprising an annular portion 42, an elbow portion 43, and a transfer portion 44, with a sequence of melt passageway portions 48B, 48C, and 48D configured therethrough, respectively, for fluidly connecting the discharge end of the barrel 40 to a melt passageway 48E of an injection assembly distributor valve 45.
The typical injection assembly 29 includes the distributor valve 45, a machine nozzle 46, a shooting pot 52, and a shooting pot piston assembly 53. In particular, the distributor valve 45 includes a melt passageway portion 48E which is configured for alternately connecting, by controllably orienting a valve spool 47, a melt accumulation chamber 48P, configured within the shooting pot 52, with either the melt passageway 48D of the barrel head 41, for charging the accumulation chamber 48P with the molding compound, or to a melt passageway 48F that extends through the machine nozzle, for a subsequent injection of the accumulated melt into the melt passageway 48G of the mold sprue. The shooting pot 52 includes, from front to back, an annular head portion 57, a cylinder portion 55, and a head of a piston 54 that is configured to reciprocate within a bore of the cylinder portion 55. The accumulation chamber 48P is the volume of the cylinder 55 in front of the piston 54, and also includes a tapered passageway that extends through the annular head portion 57. The accumulation chamber 48P is obviously variable in volume, dependent on the position of the piston 54. In operation, the piston 54 is forced to retract within the cylinder 55 when the accumulation chamber 48P is being charged, during the step of melt transfer from the extruder, and subsequently the piston is forced to advance into the cylinder to empty the accumulation chamber 48P, during the step of injection. The piston 54 is a member of a shooting pot piston assembly 53 that also includes an injection cylinder 56, as is commonly known.
The barrel 40, barrel head 41, distributor valve 45, shooting pot 52, and the machine nozzle 46 each configured to include heaters 50 arranged therealong, as shown with reference to FIG. 2B, for a controlled heating, in use, of the molding materials along the screw and melt passageway portions 48A, 48B, 48C, and 48D, 48E, 48F, as well as the accumulation chamber 48P, as is commonly known.
With reference to FIG. 2A the configuration of the barrel 40 and extruder screw 60 can be seen in detail. The extruder screw 60 includes a cylindrical shank with an outer surface providing a screw root 62 around which extends a helical screw flight 61. With reference to FIG. 2B (note: the variation in the root 62 diameter has been exaggerated and the screw flight 61 omitted for illustrative purposes), it is shown that the diameter of the screw root 62 varies over the length of the extruder screw 60 to provide the stages of melt feeding, compression/melting, and metering, as commonly known. The screw is also shown to include a conical shearing portion 68 followed by a cylindrical shearing section 69, both of which function to assist in establishing a boundary between the adjacent extruder stages to assists in a downstream venting of volatile elements, as commonly known.
Again with reference to FIG.(s) 2B, the boundaries and extent of each of the extruder stages S1, S2, S3, S4, S5, S6, S7, and S8, and the barrel thermal control zones T1, T2, and T3, are shown as configured for the sake of the experiments undertaken with the compounding materials as listed in Table 1, which were starve fed into the barrel 40 at a throughput rate of about 180 kg/h:
TABLE 1Second CompoundingFirst Compounding Material (Blend)Material68% PP Montell VM61002% Polyblond 320030% glass fiber(EC 15-12 P368)68% PP Montell H32GA2% Polyblond 320030% glass fiber(EC 15-12 P368)Table 1
Accordingly, the extruder screw 60, the extruder stages S1, S2, S3, S4, S5, S6, S7, and S8, and the barrel thermal control zones T1, T2, and T3 were configured as listed in Tables 2, 3, and 4, respectively:
TABLE 2Total screw length (L/D):25Screw diameter (mm):100Length of first section (L/D):16Number of flights:2Flight pitch (mm):90Flight depth feed section (mm):10.2Flight depth metering section (mm):3.9Compression ratio:2.6Annular slit (mm):2.1Length of second section (L/D):9Number of flights:1Flight pitch (mm):90Flight depth venting section (mm):13.3Flight depth metering section (mm):5.3Pump ratio:1.51Table 2
TABLE 3S1Feeding Stage (low pressure)S2Compression Stage (high pressure)S3Metering Stage (high pressure)S4Conical Shearing Stage (high pressure)S5Cylindrical Shearing Stage (high pressure)S6Venting and Entrainment Stage (low to ambient pressure)S7Re-Compression Stage (high pressure)S8Metering Stage (high pressure)Table 3
TABLE 4T1First Barrel Thermal Control Zone at 250° C.T2Second Barrel Thermal Control Zone at 240° C.T3Third Barrel Thermal Control Zone at 230° C.Table 4
The experiment was conducted using the in-line compounding molding system 10 described above whilst operating an extrusion/injection molding process that included the steps of:                (i) configuring a first contiguous melt passageway between the extruder assembly 38 melt passageway portions 48A, 48B, 48C, 48D, the injection assembly melt passageway portion 48E, located in the distributor valve 45, and the accumulation chamber 48P, located in the shooting pot 52;        (ii) extruding the molding compound, through the first contiguous melt passageway, for charging the accumulation chamber 48P with a required volume of the compounding material by substantially simultaneously:                    a. providing an inflow, starve fed, of the first compounding material into the screw passageway 48A, of the barrel 40, through the primary barrel feed throat 58;            b. providing an inflow, starve fed, of the second compounding material into the screw passageway 48A, whilst venting volatiles from the first compounding material, through the downstream auxiliary port 59;            c. rotating the extruder screw 60 to convey the first and second compounding materials through a plurality of extruder stages;            d. heating the barrel 40 to attain and thereafter sustain a processing temperature of the first, second, and ultimately the composite compounding materials;                        (iii) reconfiguring the melt passageway to isolate the melt passageway portions of the extruder assembly 38 and to configure a second contiguous melt passageway between the injection assembly accumulation chamber 48P, the melt passageway portion 48E and the melt passageway portion 48F, located in the machine nozzle;        (iv) injecting a portion of the compounding material in the accumulation volume 48P, through the second contiguous melt passageway, into the melt passageway 48G, located in mold sprue 85, for filling the molding cavity 83 of the at least substantially closed injection mold;        (v) substantially simultaneously reconfiguring the first melt passageway for recharging of the accumulation chamber 48P and the cycling of the mold clamp unit 12 to effect the removal of a molded article from the injection mold in preparation for a subsequent molding cycle.        
The results of the experiment revealed the problem, introduced hereinbefore, wherein the lack of screw flight and shank wiping in the venting and entrainment stage S6, adjacent the auxiliary port/vent 59, caused significant melt flow problem therethrough. In particular, it was observed that the first compounding material (i.e. polypropylene blend) glued on the screw root 62 without, or with occasionally intermittent, axially feeding by the screw flights 61.